It is well-known that the processes of plasma chemical vapor deposition, or plasma "CVD," and plasma etch occur within a reactor. One exemplary prior art reactor is described in U.S. Pat. No. 5,273,588, entitled "Semiconductor wafer processing reactor apparatus comprising contoured electrode gas directing means." Typically, the reactor of the prior art has a pair of electrodes and a radiofrequency source which, in combination, generate a discharge between the electrodes to ionize reactive gases therein. These ionized gases form a "plasma" which deposits film onto, or etches film off of, surfaces in contact with the plasma.
In plasma CVD, for example, a workpiece, e.g., a semiconductor wafer, is clamped to one of the electrodes so that selected films can be deposited onto the workpiece's surface(s) exposed to the plasma between the electrodes. Successive exposures to differing plasmas can create desirable semiconductor films on the surface, such as a bilayer of titanium and titanium-nitride. Similarly, in plasma etch, coatings or films can be removed selectively when exposed to plasmas formed by appropriate etch gases, e.g.,carbon tetrafluoride.
Typically, the electrical discharges which transform the reactive gases into plasma are generated by applying differential voltages to the electrodes. These discharges are usually radiofrequency ("RF"), as generated by an alternating current ("AC") RF source connected to the electrodes. A low-frequency, direct current ("DC") source can also be used to create plasma by generating an ionizing "spark" within the gap between the two electrodes.
To facilitate the application of AC and/or DC voltage differentials to the electrodes, one of the electrodes is usually grounded and the other is held at potential, i.e., at high voltage. The workpiece can be connected, mounted or clamped to either electrode so that at least one surface of the workpiece receives the desired process of film deposition or etch. However, because it is often desirable to heat the workpiece during the plasma CVD or etch process, one of the electrodes also generally functions as a heated carrier. This additional process of controlling electrode temperatures during film deposition or etch is extremely difficult unless that electrode is held at substantially ground potential. Such control is particularly challenging in order to attain the desired accuracies of +/- two degrees for electrodes of up to seven hundred degrees Centigrade.
Accordingly, the prior art recognizes the desirable configuration in which the workpiece is connected to a heated carrier electrode that is grounded; and in which the other electrode is attached to high voltage, e.g., fifty to three-thousand volts, AC or DC.
The prior art further recognizes that the high voltage electrode is conveniently arranged with a plurality of gas outlets extending therethrough so that reactive gases may be injected through the outlets and between the electrodes so as to uniformly distribute gases over the surface of the workpiece. These outlets are thus a series of holes extending through the electrode and connected to the appropriate gas source. In the prior art, the high voltage electrode in this configuration is sometimes referred to a "showerhead" electrode because of its appearance and similarity to a common bathroom showerhead.
The showerhead electrode of the prior art includes an inner mixing region, such as illustrated in FIGS. 1 and 1A. The mixing region is formed within the electrode and between a gas inlet and the several gas outlets. More particularly, the gas inlet leads to an inner volume which facilitates the mixing of gases therein and the distribution of gas from a single inlet to a plurality of outlets. The gases then exit from the inner volume through the outlets for ionization within the gap.
The operation of the prior art reactor with a high voltage showerhead electrode is straightforward. Specifically, the electrodes are energized by the source so that a stable and uniform plasma is created between the two electrodes. A uniform plasma discharge is desirable to achieve uniform plasma etch characteristics and/or the deposition of high-quality, uniform CVD films.
Nevertheless, one problem with the above-described prior art reactor is the generation of undesirable discharges within the showerhead electrode while gas is injected therein. These discharges, for example, are created within the gas inlet which connects between ground and the high potential of the showerhead electrode. Once the reactor is energized, gas molecules which are injected within the inlet are susceptible to ionization by high energy electrons, i.e., those electrons which have sufficient energies to ionize many such gas molecules. This ionization results in plasma which can deposit unwanted films onto reactor surfaces, and which can cause overheating damage to reactor components due to the extreme power densities in reactive plasma.
As illustrated in FIG. 1, the prior art solution to reducing or eliminating unwanted electrical discharges in the gas inlet is to utilize a ceramic (dielectric) RF feedthrough. This solution works on the known principal that high energy electrons cannot ignite a plasma in a gap which is very large as compared to the electron mean-free path. The ceramic feedthrough is large enough to ensure that these electrons lose enough energy through multiple collisions with gas molecules so as to suppress ignition of the gas.
However, typical plasma deposition and etch pressures are between about 0.1 and 10 Torr. For such pressures, reliable discharge suppression can occur only with large, unwieldy, expensive and exposed ceramic feedthroughs, each of which is undesirable for practical and business reasons. Such ceramic feedthroughs are also particularly unsafe for users of the reactor since the feedthroughs function to contain gaseous flows which are toxic, flammable, corrosive, explosive, and otherwise hazardous. Any break in the ceramic feedthrough can introduce reactive gases into the air surrounding the reactor, causing a situation that is dangerous to personnel and the surrounding environment.
It is, accordingly, one object of the invention to provide a high potential showerhead electrode that reduces the problems associated with the prior art.
Another object of the invention is to provide a reactor which enables plasma CVD and/or plasma etch without substantial discharge and ionization in the high voltage electrode.
Yet another object of the invention is to provide process methodology for plasma CVD and/or plasma etch which reduces contamination within the reactor, as compared to the prior art.
Still another object of the invention is to provide a showerhead electrode which reduces unwanted discharges within the gas inlet.
These and other objects of the invention will be apparent from the description which follows.